feat: Add tree editor validation UI (Workstream A complete)

Implements comprehensive validation feedback system for tree editor:

Task A.1 - Circular Reference Detection:
- Added detectCircularRefs() function in treeEditorStore
- Detects loops in both decision options and action next_node_id chains
- Prevents infinite navigation paths

Task A.2 - ValidationSummary Component:
- Created collapsible panel showing error/warning count
- Click error to select problematic node
- Color-coded: red for errors, yellow for warnings
- Icon indicators (AlertCircle, AlertTriangle)

Task A.3 - TreeEditorPage Integration:
- Added ValidationSummary component display
- Save button disabled when errors exist
- Warnings are informational only (don't block save)
- Added manual "Validate" button in toolbar
- Imported CheckCircle2 icon for validate button

Task A.4 - Visual Node Error Indicators:
- Added error/warning badges on problem nodes
- Tooltip on hover showing specific error messages
- Red ring for errors, yellow ring for warnings
- Shows count of errors/warnings per node

All tasks from implementation plan completed.
Build tested successfully.

Related: Issue #1

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/store/treeEditorStore.ts

@@ -623,6 +623,44 @@ export const useTreeEditorStore = create<TreeEditorState>()(
 
         validateNode(state.treeStructure)
 
+        // Check for circular references in next_node_id chains
+        const detectCircularRefs = (startId: string, visited: Set<string> = new Set()): boolean => {
+          if (visited.has(startId)) return true
+          visited.add(startId)
+
+          const node = findNodeInTree(startId, state.treeStructure)
+          if (!node) return false
+
+          // Check options
+          if (node.options) {
+            for (const opt of node.options) {
+              if (opt.next_node_id && detectCircularRefs(opt.next_node_id, new Set(visited))) {
+                errors.push({
+                  nodeId: node.id,
+                  message: `Circular reference detected: "${opt.label}" creates a loop`,
+                  severity: 'error'
+                })
+                return true
+              }
+            }
+          }
+
+          // Check next_node_id
+          if (node.next_node_id && detectCircularRefs(node.next_node_id, new Set(visited))) {
+            errors.push({
+              nodeId: node.id,
+              message: `Circular reference detected in node "${node.title || node.id}"`,
+              severity: 'error'
+            })
+            return true
+          }
+
+          return false
+        }
+
+        // Run from root
+        detectCircularRefs('root')
+
         // Check for at least one solution
         if (!hasSolution) {
           errors.push({